The Wavemeter Class C (Aust) is a device used for setting the receive or transmit frequency of any wireless set. The frequency range covered is 1470 Kcs to 10 Mcs, in 3 ranges. It is a small device measuring 15.25 inches by 10.25 inches by 9 inches. It is in a metal case, with a vibrator power supply inside. The case lid hinges open, and the instructions are in the lid. This is a copy of the Wavemeter Class C, which was made in England during WW2.

Wavemeter in the Case

MECHANICAL DESIGN
The wavemeter is in a metal box, with a fabric shoulder strap. Some cases were aluminium, and some were made of steel. The steel case weighs 26 pounds and the aluminium case weighs 20 pounds. The front cover is for protection, and hinges up, to expose the controls. Attached to the cover there are operating instructions, a circuit, and a component list. Normally this wavemeter is powered from a 6 volt large battery. However it can be run from a separate LT and HT power supply or batteries. There is a cable to allow the connection of a high tension (100 volts) and low tension battery (6 volts) inside the lid.

Inside the case to the left is a separate compartment, also with a cover. Behind this is a vibrator power supply. Also here, is a 2 pin cable to allow the connection to a 6 volt main battery. There is a spare valve in a metal case. This valve has a sticker on it, containing the wavemeter serial number, so the valve can be used without re-calibrating the wavemeter. There is a frequency correction chart in a sleeve. There is a round panel in the back, which can be removed to change the valve (if necessary).

Case View

Spare Valve

The box is splash proof, and has a gasket around the wavemeter electronics. The front has a removable dial window, to allow the changing of the indicator lights.

Dial and Lense

Vibrator Compartment

The wavemeter electronics are mounted on a fibre board, with the components on tag strips. There is a switch to change bands and provide a different capacitance for each band. The coil has a large diameter with a rotary wiper inside it to change the inductance. Located between the fibre board and the front panel is the tuning mechanism. This is a gear train connecting the front tuning knob to the tuning coil wiper. This also rotates a large disk with the tuning frequency engraved on it. A small lense moves up and down to shown the tuned frequency. The band switch mechanism changes the tuning capacitance. It also moves a mask to show the appropriate tuning scale. The tuning scales are: Range 1: 1470-2870 Kcs. Range 2: 2800-5520 Kcs. Range 3: 5280-10260 Kcs.

Front Panel

At the left hand side is the 6 volt input socket and the ON/OFF switch. Next is the insulated handle, which is actually the aerial. Then there is the removable dial window. To the right is the Range switch and the main tuning knob. Below this is the PHONES socket. At the bottom is a nameplate. Next to this is a white chart which allows the recording of often used frequencies to be recorded.

Correction Chart

Wavemeter Circuit

ELECTRICAL DESIGN
The wavemeter uses a 6J8G triode heptode valve. The triode is set up as an oscillator. It uses a variable inductor and a fixed capacitor, connected between the plate and grid. Different capacitances are switched in for the 3 different ranges. They are a combination of 4 fixed capacitances and a trimmer. Range 1 has an extra trimmer. A small cover can be removed from the top of the case to allow these trimmers to be adjusted.

For a receiver, there is enough coupling between the oscillator, through the valve and the associated wiring, to couple energy to the aerial handle. Set the dial to the receiver signal.

For a transmitter, the other half of the valve acts as a mixer, and the aerial handle is connected to the grid to supply the input signal. The oscillator couples inside the valve and mixes with the input frequency. There is a transformer in the plate that connects to the PHONES jack. When setting the frequency of a transmitter, a heterodyne is heard in the headphones. Tune to zero beat and read the transmitter frequency from the dial.

The vibrator unit is called UNIT VIBRATORY No.1 (AUST). This is a small self contained box, that can be used to supply the100 volts HT from 6 volts DC input. The vibrator switches the 6 volts into a transformer. The transformer secondary uses a synchronous vibrator, to rectify the 100 volts DC, and then filer out any noise using a choke and capacitors. The 6 volt input also has a choke and capacitor for noise suppression. There is a spare vibrator clipped to the side of the box.

Vibrator Power Supply

Vibrator Internal View

Vibrator Circuit

RESTORATION
I had two wavemeters, so I removed them from the cases, and piled the cases in a corner. They had not been disturbed, and still had their gaskets in place. The insides were clean and in good condition. One was tropicalised, the other was not.

Inside View, Normal (LEFT) and Tropicalised (RIGHT).

They had their input connector cut off, but had the vibrator power supply. I tuned each dial, and found one was smooth, and the other did not move at all. I sprayed some lithium grease on the mechanism and worked it in, by tuning from end to end of the dial scale. After a while the dial pointer moved up and down correctly.

The Range switches were seized and did not move at all. There is a Bakelite positive indent mechanism on the Range switch. This has 2 ball bearings and a spring which presses against a cam. In one, the grease was solid, so it was cleaned out and re greased. In the other, the Bakelite had expanded and seized the shaft and mechanism. This was spaced out with some washers.

Range Switch (conical Bakelite ident switch)

The wafer switch used to change capacitors, has been cut down to clear the coil. This seems a very strange thing to do.

Range Switch (cut down wafer at right hand side)

I added the correct plug and connected up the 6 volts DC to it. As I was using the main cable, the ON/OFF switch and the 2 pin input connector was not used for this test. I hooked up a 6 volt bench power supply. I replaced a few dial lights, and checked that the valve was lighting up.

A bench HT supply was connected, and slowly increased up to 100 volts. All the capacitors were good, and all the resistors were good! A receiver was tuned to 7 mHz and the receiver BFO turned ON. Each wavemeter could produce a heterodyne very close to the dial marking. The other ranges were also checked. Neither required alignment.

The case was sanded to remove the flaking paint, and both were painted with undercoat and a khaki top coat. One had particularly good paint and the name was stenciled across the top. This was copied and a new stencil cut. The cases were stenciled with their name and their serial number.

Making a Stencil

POWER SUPPLY
In the vibrator power supplies, I replaced five of the six capacitors. The capacitors on the vibrator contacts were replaced with 1000 volt rated components. The hard and gooey rubber shock mounts, were scraped off and replaced with new rubber grommets. The vibrators were opened and their contacts were cleaned. When re-assembled, they all ran properly and generated the correct DC voltage.

MODIFICATION
I decided to convert one wavemeter into a low power short distance transmitter, to enable me to provide a program source for my radios. It can cover the top part of the broadcast band for domestic receivers, from 1470 Kcs, and up to 10 Mcs for short wave receivers.

The triode oscillator was unchanged. The heptode was changed into a modulator. The phones transformer was disconnected from the plate circuit and an RFC substituted. The aerial was connected to the plate. The phones transformer was connected across the grid resistor. Note that the phones jack has an EARTH connection on the tip, not on the barrel. Disconnect this, and leave it floating. A program source was plugged into the PHONES jack. An input level of 0 dBm (a few volts) from a tape recorder (or another radio), will provide about 50% AM modulation. These simple modifications are all that is required.

Modified Circuit

Modification showing RFC

CONCLUSION
The Wavemeter Class C is easy to repair and operate. It has a simple circuit and components. The dial is poorly lighted at the extreme ends of the dial scale. If the lamps moved with the dial pointer, it would be better. The wavemeter is not very useful today, as accurate receivers and transmitters are available. It can be used as a BFO if your receiver has none. It can be slightly modified to become a program source for radio receivers.

REFERENCES

WAVEMETR CLASS C No.1 (AUST) 1942, Australian Military Forces

Copyright
Ray Robinson

INDEX